Many waste water streams such as those generated by chemical plants, municipal waste and waste water plants, food manufacturing facilities, industrial factories, petroleum refineries and animal farms typically contain high concentrations of organic compounds that need to be removed from such waste streams in view of increasing environmental constraints. Such organic compounds include hydrocarbons, alcohols, aldehydes, ketones, carboxylic acids and other oxygenates. In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of such organic compounds in water, whereby COD is expressed in milligrams per litre (mg/1) or parts per million weight (ppmwt).
The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by:COD=(C/FW)(RMO)(32)
Where:
C=Concentration of oxidizable compound in the sample,
FW=Formula weight of the oxidizable compound in the sample,
RMO=Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO2, water, and ammonia.
The International Organization for Standardization describes a standard method for measuring chemical oxygen demand in ISO 6060.
Organic compounds that contribute to COD can be removed from waste water streams by means of physical, chemical and/or biological and biochemical processes. The COD load is important for biological processes since the COD load determines mainly the size and operating costs of the biotreater. An often used pretreatment process to remove COD contaminants from waste water streams is to subject the waste water streams to a distillation step in which the COD contaminants are stripped off water in a distillation column and separately recovered. Such distillation processes leave, however, much room for improvement in terms of separation efficiency, energy consumption and operation stability.
In particular light (C1-C2) carboxylic acids exhibit high solubility for water and leave a distillation column with the bottom product. Depending on the design of the unit the treated water also contains longer chain (C3-C6) carboxylic acids which are highly soluble in water.
The treated water leaving a conventional water distillation typically has a COD load of around 1200 ppmwt.
A process generating substantial amounts of water is the Fischer-Tropsch process. The Fischer-Tropsch process can be used for the conversion of synthesis gas into liquid and/or solid hydrocarbons. The synthesis gas may be obtained from hydrocarbonaceous feedstock in a process wherein the feedstock, e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass, is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. Synthesis gas is produced in the syngas manufacturing unit of a GTL plant.
The synthesis gas preferably comes from steam reforming and/or from the partial oxidation of natural gas, typically methane, or other heavier hydrocarbons possibly present in natural gas (e.g., ethane, propane, butane). In a steam reforming process, natural gas is generally mixed with steam in a saturator and is passed through a catalytic bed comprising a catalyst. Synthesis gas can also be derived from other production processes such as, for example, auto-thermal reforming or the process known as C.P.O. (Catalytic Partial Oxidation). In the latter process streams of high-purity oxygen or enriched air together with desulfurized natural gas and a catalyst are used, or from the gasification of coal or other carbonaceous products, with steam at a high temperature.
The obtained synthesis gas is fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds and water by Fischer-Tropsch process. The obtained paraffinic compounds range from methane to high molecular weight modules. The obtained high molecular weight modules can comprise up to 200 carbon atoms, or, under particular circumstances, even more carbon atoms. Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidized bed reactors, such as entrained fluidized bed reactors and fixed fluidized bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
In a Fischer-Tropsch (FT) process carbon monoxide and hydrogen (ingredients of syngas) are converted into hydrocarbons and water according to the following general reaction:(2n+1)H2+nCO→CnH(2n+2)+nH2ODuring the conversion of syngas into paraffinic compounds also water is formed. This water exits the FT reactor with the hydrocarbons and needs to be separated for further treatment as a waste water stream.
Next to the formation of hydrocarbons, organic molecules containing oxygen can be formed during the Fischer-Tropsch process. These compounds are referred to as oxygenated compounds or oxygenates. Oxygenates include alcohols, aldehydes, ketones and carboxylic (organic) acids. The oxygenates leave the FT reactor
In GTL plants a substantial amount of water is produced which exits the FT reactor as a waste water stream. This waste water comprises trace metals and oxygenates. Due to the presence of trace metals and oxygenates the water requires treatment before it can be discharged. The required water treatment to remove the trace metals and oxygenates from the waste water stream requires elaborate and costly water treatment plants. These water treatment plants are also plot space intensive.
Further, in a GTL plant the COD load of the effluent water is important for the downstream effluent water treating plant and determines mainly the size and operating costs of the biotreater.
There remains a need for improved water treatment processes.